Background & Hypotheses
Factors that are important for getting many offspring need not be the same for both males and females. When they are different, such sex-specific natural selection is called sexual selection. This was described by Darwin in his second book The descent of man and selection in relation to sex (1871).

In general, what determines how many offspring a female can produce is the quality and quantity of food she can access and control, as well as her ability to avoid predators.
For primate males it is instead generally more important to get to mate with as many females as possible. (This is the pattern common in primates, though there are other animal species where sex roles are reversed or pretty much non-existing.)

Characters important for getting more food and better avoiding predators are thus expected to be more benificial to females than males. One such factor is sociality and cooperation. It is easier to find and defend food if you are cooperating in a group. The same applies for detecting and defending against predators. But socialilty gets more and more complex the larger a group gets. We therefore expected to find that the degree of female sociality should be connected with brain components utilized in handling social relationships.

Characters important for getting access to more matings are expected to be benificial to males than females. We hypothsized about three such characters. (1) Males can cooperate with each other for access to females, (2) males can outsmart each other, (3) males can fight each other. These three hypotehses result in three different predictions (numbers correspond). (1) Male sociality should be connected with brain components used in handling social relationships - just as for females, (2) the degree of male competition should be connected with brain components important in outsmarting each other, (3) the degree of male competition should be connected with brain components important in physical competition.

Results
We found that female sociality is connected by the brain component mainly important in handling social relationships, the neocortex. Male sociality was not. Thus, female sociality seems to have driven the evolution of the most important part of the brain when it comes to "thinking".

We also found that the degree of male competition was correlated with structures important in handling sensory-motor skills, autonomic functions, and neural pathways involved in aggression and aggression control. Thus, male competition seems to have driven the evolution of parts of the brain mainly handling physical functions.

The overarching conclusion from these results is that sex-specific natural selection (sexual selection) has been a crucial ingredient in shaping the primate brain.

Note that we did NOT examine sex-differences in the brain. We couldn't, because there is no data on the sizes of different brain structures of brains of known sex in primates.

Future directions
This study is on such lareg-scale brain structures that it is virtually impossible to pinpoint exactly what functions that are under selection, besides the large-scale patterns reported. The study is further on non-sexed specimens, making it impossible to more closely examine expected sex-differences.
To further analyze these questions, and others, a large number primate brains of known sex have to scanned and measured. Then we can really do interesting research!

Detailed Walkthrough of the Study

We examined the influence of three social factors on primate brain evolution
1. As a measure of the degree of male competition we used sexual size dimorphism (the body size difference between the sexes).
2. As a measure of the degree and complexity of female sociality we used female group size.
3. As a measure of the degree and complexity of male sociality we used male group size.

We examined the influence of those three factors on different brain structures
1. We checked the relationship between each of the above three variables (alone and in combination) and the size of different brain structures relative to the rest of the brain.
2. We also checked the relationship between each of the above three variables (alone and in combination) and the size of different telencephalon (cerebrum) structures relative to the rest of the brain.

These are the details of what we found regarding brain structures
1. The higher degree of male competition that is typical in a species
- the larger is that species' medulla oblongata, mesencephalon and diencephalon
- the smaller is that species' pons and telencephalon
2. The higher degree of female sociality that is typical in a species
- the larger is that species' telencephalon
- the smaller is that species' diencephalon
3. The higher degree of male sociality that is typical in a species
-· the larger is that species' diencephalon
-· the smaller is that species' telencephalon.

These are the details of what we found regarding telencephalon (cerebrum) structures
1. The higher degree of male competition that is typical in a species
- the larger is that species' amygdala,
- the smaller is that species' septum, striatum and schizocortex.
2. The higher degree of female sociality that is typical in a species
- the larger is that species' neocortex,
- the smaller is that species' hippocampus.
3. The higher degree of male sociality that is typical in a species,
· the smaller is that species' septum, schizocortex and neocortex.

Conclusions
1. Our results indicate that selection in relation to sex is an under-appreciated force in primate brain evolution.
2. While female sociality is tied to increased cognitive abilities (larger neocortex), male sociality and sexual selection on males is not.
3. Instead, sexual selection on males has favored brain structures involved in aggression (larger amygdala but smaller septum), sensory motor functions (medulla, mesencephalon, diencephalon), and autonomic functions (medulla, diencephalon).
4. These results are important because selection pressures acting on the brain have previously almost exclusively been treated as uniform in males and females.

Future directions
1. This study is on such large-scale brain structures that it is virtually impossible to pinpoint exactly what functions that are under selection, besides the large scale patterns reported.
2. This study is on non-sexed specimens, making it impossible to examine expected sex-differences.
3. To further analyze these questions, and others, a large number primate brains of known sex have to scanned and measured. Then we can really do interesting research!